Lateral-current control of coldcathode discharge devices



April 1965 D. J. BELKNAP ETAL 3,179,843

LATERAL-CURRENT CONTROL OF COLDCATHODE DISCHARGE DEVICES Original Filed Aug. 9, 1957 HF/0R ART REGION GLOI \REGION ARC REGION 7 lo" lo" m" a n s I CURRENT (AMPERES) LATERAL cunlmn now 2 i a 35.1 /30 SIGNAL 3o '42 OUTPUT SIGNAL FROM CRYSTAL [0 0: cm. non

IN V EN TORS DONALD J BELKNAP LLOYD R. CRUMP y 71/ Wt d A iforneys United States Patent 0 3,179,848 LATERAL-CURRENT CGNTRGL OF COLD- CATHGDE DISCHARGE DEVICES Donald J. Belirnap, Takoma Park, and Lloyd R. Crump,

Silver Spring, Md assignors to the United States of America as represented by the Secretary of the Army Original application Aug. 9, 1957, Ser. No. 677,413, new Patent No. 2,994,011, dated July 25, 1961. Divided and this application Dec. 14, 1960, Ser. No. 75,889

8 Claims. (Cl. 315-339) (Granted under Title 35, US. Code (1952), see. 266) The invention described herein may be manufactured and used by or for the Government for governmental purposes without the payment to use of any royalty there- This application is a division of copending application Serial No. 677,413, filed August 9, 1957, now Patent No. 2,994,011, for Lateral-Current Control of Cold-Cathode Discharge Devices.

This invention relates to cold-cathode gas discharge tubes and applications thereof. More particularly this invention relates to cold-cathode gas discharge devices having lateral-current control means.

A cold-cathode gas discharge tube usually consists of two or more electrodes enclosed in a rarefied gas, such as a mixture of argon, neon, helium, etc. The electrodes are supplied with a voltage to establish a small current through the gas medium. When the voltage is such that the current exceeds a certain value, and the resistance in series with the tube is not too large, the gas breaks down and conducts.

The utility of the cold-cathode gas discharge tube is greatly increased when control of its gas discharge characteristics is possible. In the prior art this control has been accomplished in a variety of ways. However, in the prior art only partial control was obtainable, and such control was not always predictable.

In our invention means are provided for establishing lateral-current flow from the main discharge stream occurring between the electrodes of a cold-cathode gas discharge tube. We have found that this lateral-current flow greatly afiects the gas discharge characteristics of a coldcathode tube in all regions. By providing accurate control of this lateral-current flow, we have been able to provide a vey sensitive, predictable and complete control for a cold-cathode gas-filled tube in all operating regions. Such control was never before available in the prior art even though cold-cathode diodes have been available for many years. With this increased control a wide variety of new applications for cold-cathode tubes is now possible.

The term lateral-current flow will be used i this application to designate a curent flow from the region of influence of the main discharge to a region where the ions or electrons, which have been removed from the main discharge stream, have a negligible effect on the main discharge. The term lateral-current flow will be restricted to refer only to a current flow from the main discharge stream which is sufficiently small so that the main discharge remains substantially between the two main discharge electrodes, as in the conventional cold cathode diode. The term lateral-current flow is used because it gives a good physical picture as to the type of control mechanism which is involved in this invention. It should be carefully noted that lateral-current flow as used herein is not necessarily limited to current that flows at right angles to the main discharge stream.

An object of this invention is to provide an improved method of controlling the gas discharge of a cold-cathode gas-discharge tube.

Another object is to provide an improved cold-cathode discharge device.

arrears Fatented Apr. 20, I955 Another object is to provide a cold-cathode discharge device having improved control means.

A further object is to provide a switching circuit which is triggered by the time rate of change of the applied trigger signal.

Still another object of this invention is to provide an improved trigger circuit which is triggered by the D.-C. current level of the applied signal.

An additional object is to provide a cold-cathode switching circuit in which the glow discharge can be switched both on and off while the anode voltage and the external work circuit remain fixed.

Another object is to provide a repeatable and reliable time-delay cold-cathode circuit.

A further object is to provide a cold-cathode gas discharge amplifier.

Yet another object is to provide a combined amplifier and switching device.

An additional object is to provide a cold-cathode relaxation oscillator which can be synchronized with a low power A.-C. control frequency over a wide range of frequencies.

An additional object is to provide a cold-cathode device which diFerentiates and amplifies an applied signal.

The specific nature of the invention, as well as other objects, uses, and advantages thereof, will clearly appear from the following description and from the accompanying drawing, in which:

FIGURE 1 is a simple cold-cathode diode circuit.

FIGURE 2 is a typical current-voltage static characteristic curve for a cold-cathode diode.

FIGURE 3 is a schematic representation of a coldcathode tube showing the basic lateral-current flow mechanism.

FIGURE 4 is a schematic representation showing one type of means for establishing lateral-current flow in accordance with the invention.

FIGURE 5 is a schematic representation of a circuit which may be used as the signal 39 in FIGURE 4 to provide operation of FIGURE 4 as a time -delay trigger circuit.

FIGURE 6 is a schematic representation of a circuit which may be used as the supply voltage E in FIG- URE 4.

FIGURE 7 is a schematic representation of a synchronized relaxation oscillator in accordance with the invention.

FIGURE 8 is a schematic representation showing another type of means for establishing lateral-current flow in accordance with the invention.

To understand how lateral-current flow afiects the gas discharge of a cold-cathode tube, the behavior of a simple cold-cathode gas-filled diode will first be considered. FIGURE 1 shows a cold-cathode diode It} connected in series with a D.-C. voltage E and a work circuit represented by the resistor R. The voltage across the diode it) is represented as V and the current through the diode It) is represented as I. FIGURE 2 shows typical currentvoltage static characteristic curves for a typical cold-cathode diode which may be connected as the diode it) in the circuit of FIGURE 1. The voltage is plotted on a linear scale and the current is plotted on a logarithmic scale. The exact shape of this static curve will in general be dependent on tube geometry, type of gas used in the tube, gas pressure, condition of electrode surfaces, etc. In addition, the shape of the curve in the Townsend region will be dependent on the magnitude of electron emission from the cathode or state of ionization within the gas derived from external energy sources. This is illustrated in FIGURE 2 where the dotted curve refers to operation in near darkness and the solid curve refers to operation in room light. The difference between the two is accounted for by a difference in photoemission from the cathode due to external radiation. Further description and details concerning the basic mechanisms which determine the current magnitude in various regions of the curve can be found in the literature (see M. .l. Druyvesteyen and F. M. Penning, Rev. Mod. Phys. 12 (1940)). The well-known terms for the various rcgions-Townsend region, glow region, and are region-of current magnitude are shown in FIGURE 2. The voltage values V V V and V and the current values l l I and 1.; refer to the voltages across, and the currents through, the diode it? at points 1, 2, 3 and 4, respectively. The numbers it), 19*, 10- and 1 indicate general orders of magnitude for the current in ampercs.

The dashed curves in FIGURE 2 are load lines for a particular value of the series resistor R and for two values of the supply voltage E in FIGURE 1. Because of the semi-log plot necessitated by the large current range, these load lines are curved rather than straight as they would be on a plot having a linear scale. The general shape of such load lines is constant for a given selection of scale magnitudes. The load line curve shifts to the left or right as R is increased or decreased and shifts up or down as the D.-C. voltage source E is raised or lowered.

For a supply voltage E and a series resistor R, the load line is the lower dashed curve of FIGURE 2. The intersection of this lower dashed load line and the static voltage-current characteristic curve determines the operating point of the circuit. This point is designated point 1. At point 1 a voltage V is across the diode 1d and a current I is flowing in the circuit. If now the supply voltage E is slowly raised to the value E the load line slowly moves upward and the operating point moves along the voltage-current characteristic curve to point 2 at which the upper dashed characteristic load line is tangent to the curve. A voltage V is now across the diode ill and a current i is flowing in the circuit. A further slight increase in the supply voltage E causes the operating point to suddenly move to point 3 with a sudden increase in current from 1 to 1 This current discontinuity is frequently referred to as breakdown or a firing of the diode. The voltage at which this occurs is referred to as the breakdown voltage. The current through the diode may change by several orders of magnitude, and visibly the tube may change from a dark condition to a flow. If the series resistor is sufficiently small, and if the right hand peak of the static curve is lower than the left, the discharge can pass into the arc region with even much larger currents resulting. If now the supply voltage E is gradually lowered back to E the operating point moves along the static curve to point 4 where the lower dashed load line is again tangent to the curve. A slight further reduction in the supply voltage E causes the operating point to suddenly move back to point 1 with a reduction in current from 1 to 1 This discontinuity is referred to as an extinguishing of the glow discharge and the diode voltage V at which it occurs is referred to as the threshold voltage.

This cycle, using a particular value of series resistance R, is illustrative of the ways in which the current through the diode may vary. In general, the ratio of current I change to supply voltage E change at any point on the static curve is dependent on the value of this series resistor R.

Under conditions of operation such as just described, several types of elementary processes are going on within the diode 1d. Electrons travelling under the influence of the electric field which exists between the electrodes are producing ionization and excitation. This results in the production of new electrons, positive ions, photons, and in some gases metastable atoms. Loss of charge is also occurring within the gas due to recombination. At the cathode surface secondary electrons are being emitted due to impinging positive ions, photons, and metastable d atoms if present. At both electrodes charges are being moved due to the flow of current in the external circuit.

At any point on the static curve of FIGURE 2 all of these processes, with perhaps others, are in equilibrium. The state of equilibrium at a given point is expressed in terms of the current I flowing through the tube and the voltage V across it. Any modification of the tube or circuit which changes the magnitude of one of these variables disturbs the equilibrium condition within the tube and will result in a new equilibrium condition being set up in which both the diode current I and the voltage V generally will have changed. This interdependence leads to the commonly employed method of controlling the dischargethat is, adjusting the anode supply voltage E to produce the desired current I through the tube. This may be done directly or it may be done indirectly by changing the external work circuit R.

In a variety of ways, the prior art has been able to obtain some control of the diode current without changing the anode supply voltage E or the external work circuit R. However, only partial and limited control was possible, and for the most part the results were not prodictable. Using the l ateral-current flow mechanism of our invention, we have been able to obtain a complete and accurately predictable control of a cold-cathode tube without changing the anode supply voltage or the external work circuit. Such control was never before available in the prior art even though cold-cathode diodes have been known and used in the art for many years.

FIGURE 3 is a schematic representation of the basic lateral-current flow mechanism of our invention. A supply voltage E establishes a main discharge stream 14 between the main discharge electrodes 13 and 15 of a cold-cathode tube 10 having an envelope 1?; and an onclosed gas 18. The electrode 13 serves as an anode and the electrode 15 serves as a cathode. A current I is flowing through the external work circuit represented by the resistor R. Means are provided, indicated generally by the arrow 19, for establishing a lateral-current fiow l6 :rom the main discharge stream 1 As mentioned previously, the term lateral-current flow has a very particular meaning. It is not restricted to a current flow from the main discharge stream which is necessarily lateral in the physical sense, as appears in FIGURE 3. The term lateralcurrent flow in this application is being used to designate any current flow from the region of influence of the main discharge stream 14 to a region where the ions or electrons, which have been removed from the main discharge stream 14, have a negligible effect on the main discharge stream 14. The term lateral-current flow is restricted, however, in the sense that it refers to a current ilow from the main discharge stream 14 which is sufficiently small so that the main discharge stream reaains substantially in the region of the main discharge electrodes 13 and 15. Any cold-cathode structure, in which this lateral-current flow can be produced as described above, may be substituted for the schematic representation of FTGURE 3. For most structures, the flow to from the main discharge stream 14 will have to be only a small portion of the main discharge stream 14 in order to maintain the main discharge stream in the region of the main discharge electrodes 13 and 15.

To understand the effect of lateral-current flow, consider again a point on the static curve of FIGURE 2 in which all processes occurring Within the cold-cathode tube are in equilibrium. The magnitude of current flowing between the main discharge electrodes 13 and 15 of the tube 10 is dependent on the equilibrium rate at which electrons and positive ions are produced and lost in the gas 18 and at the bounding surfaces. Any change in the rate at which charge is produced or lost Will be accompanied by a change in the current being conducted through the tube 16 and the external work circuit R. Thus, if a lateral-current flow 16 from the main discharge stream 14 is established, the previous equilibrium conditions will be disrupted and the main discharge stream 14 between the electrodes 13 and 15 will readjust itself until a new equilibrium condition is established taking into consideration this lateral-current how 16. We have found that the sensitivity of this mechanism is such that a small change in lateral-current flow 15 is able to produce a much larger change in the main discharge stream 14 and thus in the current I flowing through the external circuit. This increased sensitivity is believed to result because a considerable part of the main discharge stream 14 is indirectly obtained as a result of the secondary emission produced by positive ions striking the cathode electrode 15. Some of the ions striking the cathode 15 will cause an electron to escape because of secondary emission. Each electron which escapes from the cathode 15 increases the ionization of the gas by collision with gas molecules on its way to the anode 13. Since one electron ordinarily collides with many molecules on its way to the anode 13, a multiplied increase in ionization results. Increasing the number of ions striking the cathode 15 therefore will result in an amplified increase in the main discharge stream 14. And conversely, decreasing the number of ions striking the cathode 15 results in an amplified decrease in the main discharge stream.

Ions may be prevented from reaching the cathode 15 by establishing a lateral-current fiow of ions from the main discharge stream 16. On the other hand, more ions may be made to strike the cathode 15 by establishing a lateral-current flow of electrons from the main discharge stream 14. This increases the ratio or" positive ions to electrons in the main discharge stream 14 causing a decrease in the positive ions which recombine with electrons on their way to the cathode 15. As a result, a greater number of ions will strike the cathode 15. An amplified effect on the current I in the external circuit can thus be obtained by controlling the lateral-current flow of ions of electrons from the main discharge stream 14. These amplified changes can be obtained in any region of operation of the gas discharge tube 10.

Control of the lateral-current flew 16 from the main discharge stream 14 provides complete switching control from region to region without changing the supply voltage E or the external work circuit R. This makes it possible to conveniently initiate and extinguish the diode glow discharge as desired for any particular application merely by controlling the lateral-current flow 16. To explain how this switching control is obtained, assume first that there is no lateral-current flow 16 and the operating point is initially in the Townsend region. The supply voltage E is adjusted to have a value between the breakdown voltage and the threshold voltage as detined in connection with FIGURE 2. The resistor R is chosen to be sufiicient- 1y small to permit operation in the glow region. The above adjustments provide two stable operating points, one in the Townsend region and the other in the glow region. If a lateral-current flow of electrons is now established from the main discharge stream 14, the number of ions striking the cathode 15 will increase causing an increase in the ionization of the gas 13 within the tube 10. If this lateral-current flow of electrons is large enough, initiation of the glow discharge will occur. Since the supply voltage E is larger than the threshold voltage, the tube 10 will remain in the glow region after this lateralcurrent flow of electrons ceases. If a lateral-current flow of ions is new established, a decrease in the ionization within the tube 10 will occur. If this lateral-current flow of ions is large enough, ionization of the gas 18 will fall below the level needed to maintain the glow discharge, and the glow discharge will be extinguished, returning the operating point to the Townsend region. Since the supply voltage E is below the breakdown voltage, the tube will remein in the Townsend region after this lateralcurrent flow of ions ceases. The tube may thus be switched between the two regions. If the lateral-current flow is maintained after triggering, Switching is possible even if the tube 10 has only one stable operating point since the continuing lateral-current flow will maintain operation in the other region. This means that the supply voltage E need not be adjusted to be between the breakdown and threshold voltages, but may be considerably above the breakdown voltage. For zero lateral-current flow, operation will be in the glow region, and for a continuous lateral-current flow of ions above a minimum value, operation will be in the Townsend region. Exploratory tests indicate that similar switching control as illustrated above can be expected between the glow region and the arc region.

Within any particular region, the device of FIGURE 3 may also be used as an amplifier because changes in the lateral-current flow 16 cause an amplified change in the current I in the external circuit as explained previously. To obtain amplification, the lateral-current flow 16 is varied in accordance with the signal to be amplified. The operating region may be chosen as desired for the particular application. For use as an amplifier, the operating conditions, as determined by the supply voltage E and the external work circuit R, should be adjusted to prevent switching between regions. If operation is desired in the Townsend region, for example, this may be accomplished by making the resistor R large enough to prevent switching to the glow region. Likewise, if operation is desired in the glow region, the resistor R may be made suilicient- 1y small to prevent switching to the Townsend region and sufliciently large to prevent switching to the arc region.

It should be noted that the eiiects of lateral-current flow have been considered only in regard to cold-cathode diodes. It is not expected that this lateral-current flow mechanism will be effective in controlling heated-cathode diodes because of the different equilibrium conditions existing. In the heater-type of gas diode, a tremendous quantity of electrons is always available at the cathode. Secondary emission electrons produced by positive ions bombarding the cathode will thus have a negligible effect on the main discharge stream. Lateral-current flow, there fore, would be practically insensitive in a heated-cathode type of gas diode.

On the other hand, it has been shown that the main discharge stream 14 in a cold-cathode tube is very dependent upon the lateral-current flow 16 from the main discharge stream 14. The means for establishing this lateralcurrent flow 16 are generally indicated by the arrow 19 in FIGURE 3. FIGURES 4 and 8 are examples of two types of means for establishing this lateral-current flow 16.

In FIGURE 4, numeral 26 represents an electrode external to the gas 18 within the tube 1%). This external electrode 26 may be a conductor placed outside of, or near, the cold-cathode tube it). For example, the outside of the envelope 12 could be given a conductive coating and this coating used as the external electrode 26. The intervening air, glass of the envelope 12, and the gas 18 form the dielectric of a coupling capacitance represented by the dashed capacitor C, To produce the lateral-current flow 16 from the main discharge stream a varying voltage signal 30 is applied to the external electrode 26. Or, alternatively, the value of the coupling capacitance C may be varied such as might be produced by relative physical motion between the tube 10 and the external electrode 26. Because of the capacitive type of coupling, the lateral-current flow 16 will vary as the time rate of change of the voltage signal 30. To obtain a constant value of lateral-current How 16, therefore, the voltage varying signal 30 must rise or fall at a constant rate.

Since the lateral-current flow 16 is dependent upon the time rate of change of the signal 30, the circuit of FIG- URE 3 can be advantageously used as an amplifying and differentiating device in any desired region of operation. The signal 30 will produce a lateral-current flow 16 which varies as the time rate of change of the voltage signal 30, thereby causing an amplified variation in the current I in the external circuit. A voltage may thus be obtained across the resistor R which is the differentiated and amplified voltage of the voltage varying signal 39. Of course, the operating conditions of the circuit and the rate of change of the voltage varying signal 3% must be adjusted so that operation does not switch to another region. This may be accomplished by methods described previously in connection with FIGURE 3.

The device of FIGURE 4 may also be advantageously utilized as a special type of triggering circuit which can be triggered either on or off only by the time rate of change of the voltage varying signal and not by the ill-C. level of the signal. As described previously, establishing a lateral-current flow 16 of sufilcient magnitude, and in the proper direction, will cause switching from the Townsend region (olf condition) to the glow region (on condition), or vice versa. A rising signal 3d turns the tube on and a falling signal 3% turns th tube oil. To maintain the lateral-current flow would require that the voltage varying signal 38 be continuously falling or continuously rising as the case may be. Since such a signal cannot be practically produced for any length of time, it will be necessary to provide two stable operating points so that the operating point will remain in the region to which it has been switched after the rising or falling signal ceases. This may be accomplished by adjusting the supply voltage E to be between the threshold voltage and the breakdown voltage as described in connection with FIGURE 3.

The advantage of using the circuit of FIGURE 4 as a trigger circuit is that switching is obtained only in re sponse to the time rate of change of the triggering signal or relative motion between the tube lb and the external electrode 26. A D.-C. voltage many times that which would normally produce breakdown can thus be maintained on the external electrode 26 with practically no efiect on the main discharge stream 14. A trigger device according to FIGURE 4 was constructed using an ordinary Ne2 type of cold-cathode diode with a conductive coating serving as the external electrode 26. With this device adiusted to have two stable operating points, triggering between the Townsend region (off condition) and the glow region (on condition) was obtained in 10 to 20 milliseconds for a voltage varying signal 36 changing as slow as 5 to 10 volts per second. This is an indication of the considerable sensitivity of the lateralcurrent flow control mechanism. For large rate of rise signals switching was obtained in less than 100 microseconds. Because the device or" FIGURE 4 can conveniently be cut on or off, it is ideally suited for use as a binary element in computer circuits.

The device of FIGURE 4 also lends itself to use as a time-delay trigger circuit because of a unique feature of this device. For such use, the battery as and the switch 42 shown in FIGURE 5 are used as the signal 3% in FIGURE 4. The operating point of the tube id is chosen so that the tube is initially in the Townsend region or the glow region. The switch 42 is initially open. The time delay is triggered by closing the switch 42 which suddenly applies the negative voltage of the battery 4% to the external electrode. This produces a large negative rate of rise signal which causes the current I in the external circuit to be greatly reduced. If the negative voltage of the battery 44 is high enough, we have observed that the current I does not return to its initial value, but there is a time delay during which the current I remains at the value it was reduced to upon closing the switch :2. The length of this time delay may be a fraction of a second, several seconds, or a minute or longer depending upon the voltage of the battery 40 and the value of the coupling capacitance C. For a given negative voltage of the battery 40 and the capacitance C, we have been able to obtain repeatable time delays over a wide timedelay range.

A logical explanation for this effect may be obtained by considering the eilect of a rapidly rising negative signal on the equilibrium conditions within the tube 10. Assume that the operating point is initially in the Townsend region with an initial current I flowing in the external circuit. Application of a rising negative signal to the external electrode 2-6 causes positive ions to be drawn out of the main discharge stream 14. If a suflicient number of ions are still available in the main discharge stream 14, a new equilibrium condition will be established with a reduced value of current I resulting from this lateralcurrent flow of ions. When the negative rising signal levels oil, the tube will return to its original operating point and the current I will return to its initial value. If, on the other hand, the negative signal is so large that the rate of rise signal resulting sweeps practically all of the ions out of the main discharge stream 14, the main discharge will be rapidly extinguished due to the loss of most of the positive ions within it. The state of ionization drops to a low level and the impedance through the gas in the signal circuit greatly increases. The charging circuit for the coupling capacitance C will thus have a considerable time constant and a lateral-current flow will continue for a considerable time until the capacitance C charges to the potential of the battery ill. This lateralcurrent flow maintains the current I at its reduced value during this charging time period. When the capacitor C becomes fully charged to the potential of the battery 49, the current I returns to its initial value.

in order to have this time delay it is not necessary that the initial operating point be in the Townsend region. The operating point may be in the glow region, or the tube may be in a state of relaxation at the time the switch 42 is closed. In any case, if the ionization is swept out by the rising voltage signal, a time delay will occur which can be varied by proper choice of signal and circuit.

If the initial operating point is in the Townsend region, it is possible with suitable circuitry to cause the tube It to pulse into the glow region before returning to its initial operating point in the Townsend region. To accomplish this, the battery 54 the resistor 52, and the capacitor of FEGURE 6 may be used to obtain the supply voltage E in FIGURE 4-. The voltage of the battery 50 is chosen to be above the breakdown value of the tube Till. The resistor 52 is chosen so that the cur rent I at the operating point in the Townsend region causes the voltage E across the capacitor 54 to be between the breakdown voltage and the threshold voltage of the tube it When switch 42 is closed, the current I reduces to a very small value causing the capacitor 54 to charge to a voltage which is above the breakdown voltage. This charging should occur before the end of the time delay period. Thus, when the lateral-current flow of ions ceases and the operating point returns to its original position in the glow region, the voltage E will be above breakdown causing the tube 10 to pulse into the glow region. This type of circuit provides a tube with a very small standby current in the Townsend region, which, following a predetermined time delay, will give a sharp pulse into the glow region and then return to its initial standby condition.

Another unique application of the device of FIGURE 4 is as a synchronized relaxation oscillator, as shown in FIGURE 7. The battery '79 has a voltage above the breakdown value, and the resistor 72 is sufficiently small to permit the capacitor 74 to charge to a voltage above the breakdown value. With no signal applied to the external electrode 26, the circuit will be in a state of relaxa-- tion oscillation at a frequency determined by the circuit parameters. We have observed that application of a signal between the external electrode 26 and the cathode electrode 15 will cause the relaxation oscillation to be synchronized with the frequency of the signal over a wide range of frequencies. This synchronization is particularly advantageous when the applied signal is the output of a crystal oscillator as in FIGURE 7. Almost no power would be required from the crystal oscillator, while a signal of considerable power could be obtained across the load resistor 78 at the exact frequency of the crystal oscillator. It will also be evident that by properly choosing the circuit parameters, the frequency of the signal obtained across the resistor 76 could be either multiplied or divided by any desired small integer. It has also been observed that with suitable circuitry the relaxation oscillation can be amplitude modulated and to a somewhat lesser extent frequency modulated.

A second means of providing lateral-current flow is shown in FIGURE 8. An additional conducting electrode 66 is placed within the envelope 12 in contact with the gas 18 in a relatively weak electric field region of the main discharge stream 14. This will insure that the internal electrode does not affect the action of the main discharge stream 14. This electrode may be placed along the inside surface of the envelope 12, or, the envelope 12 itself, if made of conductive material, may be used as the electrode 66. This internal electrode 66 is provided with an external connection to which a signal 36] may be directly applied. If the switch 62 is open so that the capacitor 65 is in series with the internal electrode 66, and the bias 68 is omitted, operation of the device of FIGURE 8 will be identical with that of FIG- URE 4. That is, the lateral-current fiow 16 will be dependent upon the time rate of change of the applied signal. The device of FIGURE 8, therefore, can be directly substituted for the device of FIGURE 4 in all applications described in connection with FIGURE 4. The FIGURE 8 device, however, is more versatile since the capacitor 65, which corresponds to the coupling capacitor C in FIGURE 4, can be chosen as desired for any particular application.

If the switch 62 is closed so that no capacitive coupling is employed, it can be seen that the lateral-current flow 16 between the main discharge stream 14 and the internal electrode 66 is now identical with the current 61 in the signal circuit. Therefore, any type signal which produces a change in the signal current produces a corresponding change in the lateral-current flow 16.

Switching either on or off may be accomplished using the device of FIGURE 8 merely by providing a signal which produces the necessary current 61 in the signal circuit. The big advantage of the device of FIG- URE 8 is that the lateral-current flow 16 may be very easily maintained merely by maintaining the signal current 61 which caused triggering at a constant value. This makes it unnecessary to have two stable operating points as was required for switching on and off in FIGURE 4 and the supply voltage E can now be varied over wide limits. For example, it the supply voltage E is above the breakdown voltage, and it is desired to switch from the glow region (on' condition) to the Townsend region (otP condition), the operating point may be maintained in the Townsend region by maintaining the signal current 61 which caused switching even though the voltage E is above breakdown. Because of the added versatility of the device of FIGURE 8, the supply voltage E can be varied over wide limits with very little effect on switching response. This device of FIGURE 8, therefore, is very advantageous in a great number of applications, because there is no necessity for special aging techniques or other means to obtain tubes with uniform breakdown voltages as was required in the prior art.

A further desirable feature of the device of FIGURE 8 is that the main discharge stream 14 does not slowly die out (deionize) as in the case where the supply voltage E is removed, but because of the presence of the additional internal electrode 66 is forced out by sweeping charge out of the main discharge stream 14. This makes it possible to improve the frequency response of a cold-cathode tube and makes it usable in applications where fast response is necessary.

It is apparent that the device of FIGURE 8 is very well suited for operation as an amplifier. The bias 68 provides a D.-C. biasing current which may be used in conjunction with the supply voltage E and the work circuit resistance R to choose any desired operating point in any region. The operating point will generally be chosen so that operation will be in the Townsend region for amplification of very small currents, and in the glow region and are regions for amplification of larger currents. If the signal 30 superimposes a varying current on the bias value, an amplified variation is obtained in the current through the work circuit resistance R. The same methods may be employed as described in connection with FIGURE 4 for maintaining operation within a single region.

The device of FIGURE 8 can also be advantageously operated as a combination amplifier and switching circuit. This is possible because switching between regions can be obtained by the simple expediency of changing the D.-C. level of current flowing through the signal circuit. For this type of operation, the work circuit resistance R is chosen to permit operation in both the Townsend region and the glow region. For small variations in signal current, an amplified signal will be obtained across the resistance R. By changing the D.-C. level of the signal current, switching between the two regions may be accomplished as desired. Large signal currents which cause switching between regions, produce high power pulses across the resistance R. Those skilled in the art will find many uses for these unique characteristics obtained for this type of operation of the device of FIGURE 8.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of the invention as defined in the appended claims.

We claim as our invention:

1. A switching circuit comprising in combination: a cold-cathode gas discharge tube comprising a gas-filled envelope, two main discharge electrodes enclosed within said envelope across which a main discharge stream occurs, and an internal control electrode placed within said envelope in a relatively weak electric field region of said main discharge stream; a supply voltage and a resistance in series with said two main discharge electrodes, said supply voltage having a value above the threshold voltage of said tube, and said resistance having a value which permits said tube to operate in more than one region; and means for applying current signals to said internal control electrode to cause switching between at least two regions.

2. The invention in accordance with claim 1, there being additionally provided: a current biasing source in series with said internal control electrode.

3. The invention in accordance with claim 1, wherein said internal control electrode is a conductive coating placed on the inside of said envelope.

4. An amplifier comprising in combination: a coldcathode gas discharge tube comprising a gas-filled envelope, two main discharge electrodes enclosed within said envelope across which a main discharge stream occurs, and an internal control electrode placed within said envelope in a relatively weak electric field region of said main discharge stream; a supply voltage and a resistance in series with said two main discharge electrodes, said supply voltage having a value above the threshold value of said tube; and means for applying current signals to said internal control electrode.

5. The invention in accordance with claim 4, there being additionally provided: a current biasing source in series with said internal control electrode.

6. The invention in accordance with claim 5, wherein said resistance has a value such that said tube operates in only a single region.

7. A combined amplifier and switching circuit comprising: the amplifier defined by claim 5 wherein said resistance has a value which permits operation in more than one region, and means for controlling the level of direct current signal current flowing to said internal control electrode to provide switching between regions.

8. An amplifier comprising in combination: a coldcathode gas discharge tube comprising a gas-filled envelope, two main discharge electrodes inclosed within said envelope across which a main discharge stream occurs, and an internal control electrode placed within said envelope in a relatively weak electric field region of said main discharge stream; a supply voltage and a resistance in series with said two main discharge electrodes, said supply voltage having a value which is at every instant :2 above the threshold value of said tube; and means for applying current signals to said internal control electrode.

References Cited by the Examiner UNITED STATES PATENTS GEORGE N. WESTBY, Primary Examiner.

RALPH G. NELSON, ARTHUR GAUSS, Examiners. 

1. A SWITCHING CIRCUIT COMPRISING IN COMBINATION: A COLD-CATHODE GAS DISCHARGE TUBE COMPRISING A GAS-FILLED ENVELOPE, TWO MAIN DISCHARGE ELECTRODES ENCLOSED WITHIN SAID ENVELOPE ACROSS WHICH A MAIN DISCHARGE STREAM OCCURS, AND AN INTERNAL CONTROL ELECTRODE PLACED WITHIN SAID ENVELOPE IN A RELATIVELY WEAK ELECTRIC FIELD REGION OF SAID MAIN DISCHARGE STREAM; A SUPPLY VOLTAGE AND A RESISTANCE IN SERIES WITH SAID TWO MAIN DISCHARGE ELECTRODS, SAID SUPPLY VOLTAGE HAVING A VALUE ABOVE THE THRESHOLD VOLTAGE OF SAID TUBE, AND SAID RESISTANCE HAVING A VALUE WHICH PERMITS SAID TUBE TO OPERATE IN MORE THAN ONE REGION; AND MEANS FOR APPLYING CURRENT SIGNALS TO SAID INTERNAL CONTROL ELECTRODE TO CAUSE SWITCHING BETWEEN AT LEAST TWO REGIONS. 